BILATERAL SYMMETRY

Introduction to Bilateria

Protostome Bilateria

Subkingdom: Eumetazoa

Tissues, symmetry, digestive tract, muscles and nerves

Group Bilateria

Bilateral symmetry, triploblastic, organs and organ system

Bilateral Symmetry

Bilaterian - Organisms having Bilateral Symmetry

Animals that are similar to the front and back, upside and downside.

An animal is considered to have Bilateral Symmetry if only one plane of bisection of the midsagittal or midplane produces right and left mirror halves.

Streamlining is allowed by Bilateral Symmetry.

Encountering Resources

Most animals, including Humans, are bilaterally symmetric. They belong to the group called Bilateria.

Bilateral Symmetry is also known as Plane Symmetry.

Bilateral Symmetry may have evolved when radial animals began to move up the surface.

The oldest known bilateral animal is the Vernanimaliacula.

Examples

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Bilateria Body Design

Bilateral Symmetry

Cephalized

Sensory organs concentrated in the head

Triploblastic

Three basic tissue layers

Bilateral Symmetry

Bilateral Animal Lines

Must have evolved in Precambrian

First fossils are Cambrian

Spiralian (3rd Cleave Division is spiral)

Cephalization

The differentiation of the front/anterior end of an organism into a definite head is called Cephalization. It is considered an evolutionary advancement, and is accompanied by the concentration of nerve tissues as well as feeding organisms at the head region with the purpose of integrating actions/movements in the nervous system.

Targeting Resources

Cephalization allows the anterior end of the animal to first encounter food, predators and other external factors concerning their environment. This is because the brain as well as sensory organs can be found on the anterior end. This helps protect and alarms the organism 24/7.

Motile Bacteria

Many, however, not all bacteria is able to display motility (ex. self-propelled motion) under circumstances. There are many different types of motility:

Flagella (sing. Flagellum)

Copious Slime

Internal Structure (ex. Axial Filament)

Sessile Bacteria

Bacteria that is not freely able to move about.

Musculature

Bilaterians have complete digestive tracts along with a separate mouth and anus. (Except for some red forms of bilaterians.) They have an internal body cavity called a Coelome.

They are Triploblastic which means they have three tissue layers consisting of the: ectoderm, mesoderm and endoderm.

Compartmentalization

Compartmentalization or Cellular Compartments comprise of all the closed parts within the Cytosol of the (Eukaryotic) cell of an organism. It has 5 types namely the:

Nuclear compartments including the nucleus.

Intercisternal Space

Organelles

Cytosol

Regulated Compartments

Epithelia is maintains the regulated compartments

Coelenteron or the gut has evolved in cnidarians and has multiple functions in the taxon.

The bilaterian gut is dedicated to digestion. Therefore other spaces must assume the responsibility for the other functions.

Hemal System = new role for the connective tissue.

Coeloms

A new fluid filled cavity

It is lined by Mesothelium or Middle Epithelium

Contains a third regulated compartment

It is filled with a fluid called Coelomic Fluid

Functions as a Hydrostat, Transport, Excretion, Reproduction

Animals with coeloms are called Coelomates

Coelom Formation

Alternative Coelomic Conditions

Bilateria

Most animals are considered to be Bilaterians. They are also known to be the most familiar.

They can be found almost anywhere: seas, land, freshwater and even on air.

It is important to consider separately the small and large Biletarians.

Development: Cleavage

Radial/Regulative: Cleavage planes are either parallel or perpendicular to the polar axis of the egg.

Spiral/Determinate: Cleavage planes are oblique to the polar axis of the egg.

Bilateral/Determinate: Cleavage planes are oblique to the polar axis of the egg.

Cnidaria

Comes from the Greek word “Cnidos” which means stinging nettle

Diverse in form

Armed with stinging cells called Nematocysts; inherited from a common ancestor

Lives in the world’s oceans

Hemal System

A system of tubes and ducts which morphologically parallels that of the water vascular system in echinoderms, but lies just orally to it. Its function is not known, but it seems clear that it does not perform roles typically associated with a blood-vascular system in other animals. Possible functions ascribed to it are internal defense and endocrine, but direct evidence for either is lacking.

Bilateria Body Features

Bilateral Symmetry

Triploblasty

A Coelome

A Through-Gut

CNS or Central Nervous System